Rapid advances are occurring in various electronic devices especially display devices that are used for various communicational, financial, and archival purposes. For such uses as touch screen panels, electrochromic devices, light emitting diodes, field effect transistors, and liquid crystal displays, conductive films are essential and considerable efforts are being made in the industry to improve the properties of those conductive films.
There is a particular need to provide touch screen displays and devices that contain improved conductive film elements. Currently, touch screen displays use Indium Tin Oxide (ITO) coatings to create arrays of capacitive areas used to distinguish multiple point contacts. ITO coatings have significant short comings. Indium is an expensive rare earth metal and is available in limited supply from very few sources in the world. ITO conductivity is relatively low and requires short line lengths to achieve adequate response rates. Touch screens for large displays are broken up into smaller segments to reduce the conductive line length to an acceptable resistance. These smaller segments require additional driving and sensing electronics. In addition ITO is a ceramic material, is not readily bent or flexed, and requires vacuum deposition with high processing temperatures to prepare the conductive layers.
Silver is an ideal conductor having conductivity 50 to 100 times greater than ITO. Unlike most metal oxides, silver oxide is still reasonably conductive and this reduces the problem of making reliable electrical connections. Silver is used in many commercial applications and is available from numerous sources. It is highly desirable to make conductive film elements using silver as the source of conductivity, but it requires considerable development to obtain the optimal properties.
U.S. Patent Application Publication 2011/0308846 (Ichiki) describes the preparation of conductive films formed by reducing a silver halide image in conductive networks with silver wire sizes less than 10 μm, which conductive films can be used to form touch panels in displays.
In addition, U.S. Pat. No. 3,464,822 (Blake) describes the use of a silver halide emulsion in a photographic element to form a conductive silver surface image by development and one or more treatment baths after development.
Improvements have been proposed for providing conductive patterns using photosensitive silver salt compositions such as silver halide emulsions as described for example in U.S. Pat. No. 8,012,676 (Yoshiki et al.). Such techniques involve the treatment using hot water baths containing reducing agents or halides.
U.S. Pat. No. 7,943,291 (Tokunaga et al.) describes photosensitive materials that can be used to prepare conductive silver-containing films. One or more layers for example an outermost protective layer can include various conductive fine particles such as metal oxide particles in a binder.
Thus, it is known to provide conductive silver patterns on transparent films and to put protective non-photosensitive overcoats over those conductive silver patterns. While known protective non-photosensitive overcoats provide physical protection for the conductive patterns, they also act as an insulating barrier between the conductive pattern and external electrical contacts used in a various display devices. Such insulating properties can render display device manufacture unreliable.
Thus, there is a need to create conductive patterns in conductive articles designed for display devices in which a protective non-photosensitive overcoat has finite electrical conductivity only in the direction perpendicular to the surface of the conductive article used as a touch screen sensor film. Such a protective non-photosensitive overcoat would enable effective electrical contact between external electrical circuits and underlying conductive silver patterns. Conductivity in the two in-plane directions must be avoided to prevent cross-talk between circuitry elements. It with these needs in mind, that the present invention was discovered.